Slats are devices on the leading edge of a wing which are deployed during take off and landing to increase the lift of the wing. During landing it is desirable for the slat to be fully deployed for maximum lift. Also, during landing it is desirable to open up a small slot between the slat and the wing leading edge, allowing a small amount of high-pressure air from the lower surface to reach the upper surface, where it helps postpone the stall. However, during take off, whilst it is desirable to deploy the slats at least partially to increase lift, it is preferable that there is no slot between the slat and the wing because this increases drag.
A so-called “sealed slat” seals against the leading edge when in its retracted (cruise) configuration. The slat is carried on a track which is shaped so that the slat follows the curve of the leading edge as it moves to its take off position and thus remains sealed against the leading edge. A problem with such an arrangement is that the track must have a high curvature matching the curvature of the leading edge. This makes the track and associated roller bearings bulky and thus difficult to fit within a shallow leading edge. Also, achieving and maintaining the aerodynamic sealing requirements is a major challenge.